mentor Constantijn Huygens wrote to Robert Hooke about Leeuwenhoek as "exceedingly curious and industrious"

August 8, 1673

Huygens lived only a few miles up the Vliet from Delft, so he must have visited Leeuwenhoek and read the draft of the second letter that Leeuwenhoek would send to Oldenburg a week after this letter that Huygens wrote to Hooke. Thus, this letter functioned similarly to the cover letter / character reference that de Graaf had written the previous spring. De Graaf would die later that month and be buried on August 21.

Note the reference to sections ("slice or film"), Leeuwenhoek's early solution to the problem of lighting specimens so close to his lens. Note also Leeuwenhoek's concern with movement of fluids in living bodies, which would occupy him off and on for the rest of his career.

The transcription of Letter 6909 in Briefwisseling van Constantijn Huygens is missing a phrase. Editor J.A. Worp omitted the phrase "‘a most ‘αγεωμέτρητός philosopher’" after "a person unlearned both in sciences and languages". It means "a very non-geometric philosopher". According to "Huygens en van Leeuwenhoek" by Ad Leerintveld,

Huygens quotes the proverb chiseled above the entrance to Plato's Academy: 'Αγεωμέτρητός μηδείς είσίτω', ('Let no one enter here without geometrical knowledge'). According to Plato, all science begins with mathematics, including arithmetic, geometry, astronomy and harmony. Huygens believes that Van Leeuwenhoek therefore has no access to science. He is not a philosopher, but a curious, handy craftsman. We should therefore not take Huygens' designation 'αγεωμέτρητός' literally, as if he believes that Van Leeuwenhoek, who was trained as a surveyor, by the way, would know no mathematics. With his scholarly quote in Greek, Huygens emphasizes the gap between his own and Hooke's learned science and the innate curiosity and zeal of the craftsman Van Leeuwenhoek. (my translation)

My thanks to Ineke Huysman of the Huygens Institute for drawing my attention to this omission.


Letter 6909 to R. Hooke. Brieven Constantijn Huygens 1608-1687

The text of this letter, written by Huygens in English, pertaining to Leeuwenhoek (his italics; my boldface):

Our honest citizen, Mr. Leewenhoeck -- or Leawenhook, according to your orthographie -- having desired me to peruse what he hath set down of his observations about the sting of a bee, at the requisition of Mr. Oldenbourg, and by order, as I suppose, of your noble Royal Society, I could not forbear by this occasion to give you this character of the man, that he is a person unlearned both in sciences and languages, but of his own nature exceedingly curious and industrious, as you shall perceive not onely by what he giveth you about the bee, but also by his cleere observations about the wonderfull and transparent tubuli appearing in all kind of wood, though most evidently and crystallike in the ordinary white and light wood used for boxes.

His way for this is to make a very small incision in the edge of a box and then tearing of it a little slice or film, as I think you call it, the thinner the better, and getting it upon the needle of his little microscope -- a machinula of his owne contriving and workmanship -- he is able to distinguish those tubuli so perfectly, that by these meanes he lighted upon the consideration of those valvulae you see him reasoning upon, and which indeed do discover themself in a perpétual and very pleasant series, so that, for my part, looking upon this fine fagot of crystal pipes, which in this particular is a fitter way then that of objects laying under a standing microscope, I am easily persuaded of what he discourseth further touching the easy rising of the sap by those short degrees to the highth of a tree, which otherwise could hardly be forced up but by a scarce imaginable power.

I make no question, Sir, but that since the first publishing of your excellent Physiological micrographie, you have stored up a great many of new observations, and that this also may be one of them, but howsoever I trust you will not be unpleased with the confirmations of so diligent a searcher as this man is, though allways modestly submitting his experiences and conceits about them to the censure and correction of the learned, amongst whom he hath reason enough to esteem yourself beyond all in this kind of philosophie, and many other besides. So that now he doth expect your instruction upon the difficultie he doth find in his glas pipes, whereout I am sure you are to extricate him by very good and ready solutions.